There exists a type of container hoist called a hook hoist which includes an L-shaped hoist arm pivotally mounted at the rear of a trailer. A part of the arm called a "jib" is telescopically extendable and has a hook mounted to the outboard end thereof. The arm is connected at its end opposite the jib to the trailer via a tilt frame such that, when unlocked with the tilt frame, hydraulic cylinders pivot the hoist arm back approximately 150.degree. where it may engage with a container and be pivoted forwardly, thereby lifting the container up onto the vehicle chassis. Extension of the jib pulls the container forwardly to the desired transport position. When the hoist arm is locked with the tilt frame, actuation of the same cylinders causes the hoist arm and tilt frame to pivot as a unit about the tail end of the trailer, thereby dumping the container mounted thereon. The locking mechanism is designed so that the hoist arm is automatically, mechanically unlocked from the tilt frame upon retraction of the telescopically mounted jib.
If the mechanism locking the hoist arm with the tilt frame is disengaged while the two are pivoted upwardly with a container, the whole assembly may come crashing down with considerable damage to hoist and container. Various safety mechanisms have been designed to prevent this occurrence. In one such mechanism a hydraulic interlock mechanism precludes hydraulic operation (i.e., extension or retraction) of the jib whenever the hoist-arm-pivoting cylinder assembly is not Fully retracted, thereby precluding the possibility of accidently unlocking the hoist arm from the tilt frame while the hoist arm is pivoted. Unfortunately, this also precludes extension of the jib at the conclusion of a container unloading sequence, such extension stroke being useful to drive a container onto a loading dock.
What is needed is a hoist mechanism which provides the dual loading/unloading and dumping capability and which includes a safe interlock mechanism to allow for extension or retraction of the hoist arm at any time where the hoist arm and tilt frame are locked together and pivoted away from horizontal, without the danger that hoist arm and tilt frame will accidentally become disengaged.